The substitution of transparent components of mineral origin with transparent components made of moulded plastic material provides a functional alternative at low production cost, and complete independence from certain supply sources of up-market transparent materials.
However, moulded transparent parts are, on the one hand, relatively thick since it is difficult to mould very thin parts with adequate reproducible quality. On the other hand, the wear resistance, and particularly scratch resistance of these known plastic materials is very mediocre. The use thereof for the external components of timepieces—the middle part of the case, bezel or crystal—is thus limited in time.
This low wear resistance also makes it impossible to produce components with sharp angles, which would become blunt too quickly over time on contact with the user, his clothes, and ordinary everyday objects.
It is useful, in horology, to have components having a high surface hardness, in particular for those external timepiece components, which are subject to stresses from the environment and the user. These components must also have certain physical properties of elasticity, rigidity or conversely flexibility, or particular optical properties, to enhance certain displays or components, or conversely to conceal certain parts of the timepiece.
JP Patent Application No 59052626A in the name of SUMITOMO describes the fabrication of an insulating case, wherein a synthetic resin body is first injection moulded on a rotating tool in a first position and then subjected to metal plasma sputtering in a second position of the tool, and finally undergoes another resin injection above the metal layer, on the opposite side to the basic body, in a third position of the rotating tool, opposite to the first position.
JP Patent Application 1114405A in the name of TIGERS POLYMER describes a similar system, where the metal is in the form of a film held by adhesive between two resin layers, the sequence being performed with a similar rotating tool.
US Patent Application No 2010/1310697A1 in the name of STEINEBRUNNER describes an injection moulding machine for producing a part in two injected polymer half shells, which are assembled to each other on the same machine. The machine includes an injection tool rotating in two opposite directions of rotation.